Known systems for maintaining error-free transmission of multiplexed data under varying noise conditions generally operate to vary entire frame formats rather than formats of individual data channels in frames. For example a system disclosed in U.S. Pat. No. 3,534,264 to Blasbalg et al operates to vary the code and bit rate formats of data in multiplex frames; each format change being indicated by control signals sent in the frame preceding the change. The receiver/demultiplexer responds to the control signals and adjusts its reception to the changes in transmission format. When a transmission format change involves a change in bit timing the receiver adjusts its demodulation circuits to adapt to the varied bandwidth of the data and its bit reception clocking to adapt to the altered bit timing.
This may be inefficient inasmuch as some data may be less affected by noise than other data. For instance digitized voice telephone samples may be received intelligibly in the presence of noise which can render much computer process data useless.
The present invention operates more efficiently by varying data formats in multiplexed channels on a channel-selective basis taking into account the nature of the data being sent in each channel, its vulnerability to noise and the channel capacity available for transmission. Control signals accompanying the data enable any receiving station to distinguish the format of data in each transmitted channel. By arranging these control signals to designate various destinations of data in the channels an added measure of efficiency is realized. By encoding these control signals invariably in an error correction code, regardless of noise conditions, a more reliable system is achieved.
Considered in relation to TDMA (time division multiple access) systems of the type described in U.S. Pat. No. 4,009,344 to D. Flemming, the technique described in the Blasbalg et al patent has other shortcomings which are avoidable by the present invention. In such TDMA systems bursts of time multiplexed data originated by multiple exchange stations interleave in time at a satellite repeated and return to all stations as a time division composite of bursts. Individual bursts may contain multiple data channels having various origins relative to subscriber ports of their respective transmitting stations and various destinations relative to subscriber ports of the receiving stations. Each station has buffer storage capacity only for received data destined for exchange transfer to its associated ports. Such data usually represents only a small fraction of the total channel capacity in the received composite. Therefore it must immediately be distinguished and extracted while the composite is being received.
In the system described in the Blasbalg et al patent one station receives and demultiplexes all multiplexed data transmitted by another single station. In response to varying noise detected by the receiving station, and control messages communicated back to the transmitting station, the transmitting station varies its frame format; indicating each format change to the receiving station by an advance control signal. If this technique were used in a TDMA system environment a first station receiving and extracting one channel out of a multiple channel burst sent by a second station could influence the second station to modify its entire burst format merely to protect the one channel handled at the first station. In this same example if the first station is also receiving and extracting fractional parts of bursts sent by third and fourth stations the first station could also potentially influence the burst formats of the second, third and fourth stations to the potential detriment of communications between those stations and many other stations.
The present invention avoids such potentially inefficient use of system bandwidth by adapting the stations to be able to alter data formats in individual channels of associated transmission bursts on a selective basis. The data coding in the channels is varied without changing bit rates so that reception demodulation and bit clocking adjustments are unnecessary.